BebasBanjir2015

Gorontalo

Field Reconnaissance Report on  Gorontalo Urban Flood Protection areas

Hitoshi BABA

Date: 30-31 March 2009

Name: Dr. Hitoshi BABA, JICA Expert on Water Resources Policy

Place visited: Downstream courses and some tributaries of Bolango River and Bone River, the Urban Flood Control Project areas of Gorontalo City and Lake Limboto.

01

Route map of the field reconnaissance

 

Background:

Gorontalo is a new province established in December 2000, separating from North Sulawesi Province, as the 32nd province of the Republic of Indonesia.

The southern part of Gorontalo City, middle reaches of the Bolango River, Limboto and Isimu-Pohu areas, and western area of Lake Limboto are major areas suffering from flood and sediment disasters. Among these, problems are more serious in the southern part of Gorontalo City near the confluence of the Bolango and Bone rivers.

In response to a request from the Government of the Republic of Indonesia, the Government of Japan, through JICA, conducted the Study on Flood Control and Water Management in Limboto-Bolango-Bone (LBB) Basin on between June 2001 and December 2002.

The Study outlined the Flood Mitigation Master Plan comprising of structural measures (river improvement, floodway and Lake Limboto management) and non-structural measures (watershed management and flood plain management). The Study also selected the Priority Projects: 1) River Improvement of the Lower Bone-Lower Bolango rivers 2) Tapodu River Improvement with Tapodu Gate 3) Construction of Tamalate Floodway 4) Sediment Trap Works in Lake Limboto, and Non-structural measures for intensive implementation. Feasibility study for the above selected Priority Projects evaluated those to be technically feasible and economically viable. EIA was carried out with conclusion of that the Priority Project was evaluated to be valid for natural and social environment. The EIA Evaluation Committee of Gorontalo Province has already issued a written approval on the EIA.

Based on the feasibility study and EIA of the priority projects, the Government has decided to take Borango River Improvement Project as one of the Urban Flood Control System in Selected Cities under Japan’s ODA Loan project through JICA.

The Loan agreement has already been signed between the both Governments on the end of March 2009 and the project is about to start.

Objectives:

  • To see and understand the present conditions and problems of the LBB River basin and Gorontalo flood affecting areas.
  • To make recommendations for the relevant authority who administers the above rivers and water resource management systems.

Resulted findings and Recommendations:

Flood Problems in Gorontalo

  • The most flood prone area is shown as following map. This low lying area is located at the confluence of Bolango River into Bone River right where the tributaries’ water level is affected by the Bone River flood level and tidal water level. The Bolango River is streaming by two ways in the densely populated area where existing flood protection walls and facilities are functioning only for normal tidal flood or very small scale of river flood. The walls seem to be not strong enough to bear against higher water level and have damaged in some part.
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    03

 

  • The Priority project to be implemented under the Urban Flood Control System Improvement in Selected Cities seems to be reasonably designed in consideration of those flood prone circumstances. Normalization of river streams, strengthening and newly construction of river banks and revetment works are planned.
  • The project should be implemented with community participatory in which they can share real time flood information and can aware the importance of watershed management and flood plain management. People in upstream of Bolango River and in surroundings of the Lake Limboto will have casting rolls in discussing flood control with whom in city area.
  • Consideration on excessive river discharges must have attention by river administrators and planning engineers because the design flood return period is considerably small under this project and recent climate change is getting serious impact on intensifying heavy rainfall and unexpected runoff.

Lake Limboto, the Key of Flood Control

  • Since the stream capacity of downstream of Bolango River in urban area is very small (200m3/s after completion of the project), we have to consider the best way of controlling flood water somewhere in upstream. Although some flood retention, including reservoir by constructing dam and retarding areas in middle stream of the Bolango River, can be effective for controlling flood, the Lake Limboto is the largest and most effective flood water reservoir in the watershed. We should use and conserve this precious natural reservoir as long and much as possible.
  • The master plan of LBB Flood Control and Water Management designed the Tapodu River Improvement with the consideration of flood retarding function of the Lake Limboto. The Tapodu River Improvement consists mainly of new channel construction, discharge control gate and lake side protection dikes as shown here.

    04

  • According to the master plan, flood discharge of the Bolango River is planned to be led to Lake Limboto to reduce discharge in the Lower Bolango River. A control gate is constructed across the Tapodu River near the confluence with the Bolango River. On the other side, water level of Lake Limboto will be controlled within the range from +4.00m, MSL at the lowest and +5.50m, MSL at the highest. Basically the gate passes the 550m3/s of flood discharge from the Bolango River when 750m3/s of flood discharge occurs. Design high water level of the Bolango River is set at +6.10m, MSL at the confluence of the outlet channel of the lake (the Tapodu River) with design riverbed +2.00. During flood of the Bolango River, flow direction of the Tapodu River is from the Bolango to the lake to store the floodwater, while the flow direction during ordinary time is opposite to drain the stored water. Therefore, the Tapodu River and Tapodu Gate must be designed considering the flows under both directions.

    05

  • Dikes are constructed on both banks of the Tapodu River to protect surrounding lakeside areas from the Bolango floodwater using excavated earth from the channel. As to the lake water level, the influence of the Bolango flood is slight. Lake water level varies mainly due to the runoffs from its own tributary. On the other hand, the Tapodu River improvement contributes much to the lakeside farmlands, lowering the peak water level and shortening the duration of inundation. The existing north and south Tapodu channels will serve for water supply, fish way and drainage in the areas protected by dikes, providing sluice gates at the heads and tails of the channels.
  • The Plan of priority projects has an essential concept of flood water retention in the Lake Limboto and reducing flood discharge at Lower Bolango River. However, from my experience of river engineering for disaster prevention and mitigation, I cannot stop worrying about some negative impacts and apprehensions of the plan as:
    • Risk of over discharging in the Lower Bolango River . The Plan calculates the design discharge of Bolango River of 750, 550, 390, 200m3/s for 20, 10, 5, 2 year return period respectively. The control gate passes at most the 550m3/s of flood discharge from the Bolango River when 750m3/s of design high flood discharge occurs. While in the Lower Bolango section, where the maximum stream capacity is 200m3/s, the rest of 200m3/s flood discharge is planned to pass through. However, when excessive flood discharge occurs on Upper Bolango River, the risk of over discharging in the Lower Bolango River and inundation in city area will be realized. Since the design return period is 20 year in maximum, the excessive flood that is larger than design high discharge will possibly happen to be occasionally.
    • Restricted function of flood intakes from Bolango River into Tapodu River. The Lake Limboto water level is planned to be controlled within the range from +4.00m, MSL at the lowest and +5.50m, MSL at the highest. Design high water level of the Bolango River is set at +6.10m, MSL at the confluence of the outlet channel of the lake (the Tapodu River) with design riverbed +2.00. The hydrograph in this case is shown as following chart. When the Lake water level is high due to runoffs from its own tributary, the flood control function is restricted due to low water level difference between the Lake and the intake point of Bolango River. For example, even at the design high flood of 20 year return period and the Lake water level starts from +4.4m MSL, the inflow from the Bolango River recedes after the Lake Water Level excesses over +5.2m MSL. That means, if the Lake Water Level was higher than this, the function of flood diversion would be limited. In case of medium or small scale return period of floods, this tendency will further be intensified and there will be cases of “no flood control”.

      06

    • Difficulty in maintaining the design riverbed by +2.00m MSL at the confluence of Tapodu River and Bolango River. The function of flood diversion into the Lake Limboto depends on the designed alignment and the longitudinal and cross sectional profiles of relevant channels. The design river bed by +2.00m MSL at the confluence of Tapodu River and Bolango River holds a key of hydrological system of flood control. It is, however I understand in Indonesia, quite difficult to maintain this river shape for many years because of erosion, excavation, siltation, illegal encroachment of river space and limited resources in kind and in engineering capacity.
    • Difficulty in operating and maintaining the new type gate, made of rubber and powered by air pressure, in certain operational frequency within limited budget and human capacity. The control gate is designed to be operated for flood control and Lake Water Level control. This will be a busy task for administrator of the water system of LBB. They will have to negotiate with many relating stakeholders including water users, lake side dwellers and living people in flood prone areas. I’m afraid the operation rule/manual of the gate would be very much complicated one that takes gate’s functions and operation frequency into consideration. In addition, the new type gate needs to get intensive and perfect maintenance service to secure its working. To be frank, I saw many evidences of gate facilities in Indonesia that have not working any longer.
    • Impact of the control gate on environment especially of difficulty in fish passage . Although the EIA of this project has already been approved, the control gate has some impact on river environment especially of difficulty in fish passage and aquatic habitat that have relation with the large scale of watershed ecosystem. The Lake Limboto is now connected with sea water through the Bolango River.
  • After making project site reconnaissance, I have thought of an alternative plan of flood control that could give solutions to the above apprehensions in mind.
  • The alternative plan is just a preliminary idea to be taken into consideration when the detail study and designing works be commenced. The current priority project under the ODA Loan is therefore have nothing to change and should be implemented as designed.
  • The new idea is based on a concept that the most priority area of this watershed is the city area of Gorontalo, the surroundings of the Lower Bolango River, where we should put the best effort of protection from flood. The Lake side area and upstream of the Bolango River are the next priorities instead, and thought to be conserved as much as possible. I also take the existing function of Naturally Controlled Outflows through the Tapodu River into consideration.
  • Summarizing with other points considered, followings are the basic concepts of the alternative plan:
    • Improved flood intake condition
    • Safety of Lower Bolango area first
    • Function even against excessive large scale of flood
    • Make use of existing Natural Flood Control function of the Tapodu River
    • Operation free method of flood control
    • Measures adaptable against minor stream change by sediment move
    • Environmentally less impact
    • Better cost performance within life time O&M

Alternative Plan of flood control by the Lake Limboto

(Refer the figure of the plan on the following page)

  • First, in order to improve and maximize the flood intake conditions from Bolango River into Tapodu River, I have drawn the new intake point at the upstream where the master plan placed the starting point of the left embankment of the Lake. Making quick reconnaissance on the possible site of intake and embankment, I’ve got confident that the new idea is feasible in terms of engineering.
  • The intake condition will be improved due to the higher elevation difference between the intake and the Lake than that of the previous plan. The elevation of new intake point is +3.00m MSL or more, which is higher than the design riverbed at the previous intake point +2.00m MSL. This can then contribute more effective flood control function especially when the Lake Water Level is eventually high due to runoffs from the lake’s watershed.
  • The Intake Structure shall be constructed with an overflow dike on the right embankment of the Upper Bolango River, which can put a limit of the upper most discharge into the Lower Bolango River and can secure the safety of the Gorontalo city area effectively, even when excessive large scale of flood comes. Although the overflowing flood water would raise the Lake Water Level up and cause inundation to some extent moderately, it shall be the best solution under the concept of the “Safety of Lower Bolango area first”.
  • Second, in order to make use of existing Natural Flood Control function of the Tapodu River, I would like to propose to conserve the present condition of the Tapodu River as much as possible. Since the flood intake diversion channel is relocated to the upstream section, the Tapodu River doesn’t need to have large capacity for ‘reverse water stream’ nor any control facilities such as gate. The only needed is a series of cross dikes that naturally regulate flood water into the confluence with the Lower Bolango River. The cross dikes can easily be designed based on a hydrological flood simulation. Partly referred to the previous plan, the embankments that protect both river side of housing area will also need to be designed with the cross dikes.
  • The combination of existing Tapodu River alignment and the cross dikes is a simple way of flood control and having environmentally less impact on fish passing, flora and fauna. All the more, this ‘operation free design’ gives us a better Cost Performance than the previous plan which would take a remarkable life time cost mostly for the operation and maintenance of the Control Gate.
  • Third, considering the better and safer living spaces around the Lake and the River, I made the alignment of the embankment friendly with the present community distribution. Some existing roads and market places (small bazaar), where comparatively dense housing are occupying, shall be considered as communication nodes and networks to be protected and connected with each other.
  • With this concept, part of the Tapodu River is re-channelized and the embankment is designed so as to surround the nodes and community network. New bridges and road, for connecting those nodes, are planned tentatively with two major bridges over the new flood intake channel and three bridges cum cross dikes over the Tapodu River.

    08

  • Fourth, surveying the upstream of the Bolango River, I found that some irrigation barrages are still keeping rooms for sediment trap and the riverbed is almost stable. This makes me confident that the alternative plan of new flood diversion will not suffer from serious sedimentation from the Bolango River. Even if some stream changes by sediment movement occur, the channel that has good enough profile of elevation and capacity is adaptable against such changes.
  • I also found some water retarding spaces in upstream area of the Bolango River. Under proper administration of the spatial planning, those spaces could be rooms for future development and at the same time present buffer zone for flood retention in reality. The alternative plan is quite adaptable to this condition of upstream for the time being and for the future development as well.
  • Finally, this alternative plan can eliminate most of negative impacts, risks and difficulties that I’ve apprehended on the present master plan. I hope further detail study will prove this and make the project properly implemented in near future.
  • Flood intake measure under the present master plan and alternative plan is shown on next page.

    07

Strategic Flood Control

  • Flooding is unavoidable perfectly in all areas in a basin. Because the flood water cannot disappear. But we can control flood. Controlling flood is a kind of strategic management of flood water allocation. Which area should first be protected from water and where should be flooded? That’s the question of the person on strategic command.
  • Plan of flood protection can only describe limitation of designed measures that can protect specific areas from flooding not exceeding designed magnitude (designed river discharge or precipitation that has certain return period of occurrence). If we always experience river runoff which does never exceed over the plan and all protection facilities are well constructed and maintained, we don’t need to make any flood operation. But in reality, we often need flood operation and management because natural disaster occasionally happens with over loaded magnitude against our plan. Then river engineer has to consider what will happen if the larger scale of river discharge than expected comes next.
  • Considering Bolango River and Bone River, the designed magnitude of flood is not large in my book. So what will happen if large scale of flood water comes?
  • Considering the scheduled project component under the JICA Loan, we should keep focusing on the increasing risk of flood damage. As one of typical cross section’s design shown below, all kinds of river structures have their limited capacity of maximum water level and maximum strength to bear against water pressure.
  • Flood protection wall, in general, is a structure that can stand on the condition while water level doesn’t exceed the Designed High Water Level, for example. If the water level got higher over the DHWL, the wall would lose its stability, as we remember the fact that many flood walls in New Orleans were collapsed by the Hurricane Katrina in 2005. This experience tells us that flood protection wall could cause, if excessive discharge of water run, more devastating result by generating condensed energy of flush burst by highly raised water head than that of gradually increasing water head when there had been no wall.
  • The possibility of flood protection wall collapse is depending on the occurrence of higher water level than the maximum designed water level. If we take design water level increased, therefore, the possibility of collapse decreases.
  • On the other hand, the risk of flood damage is depending on the height of collapsing structure as mentioned above. Therefore the higher the wall or dike, the larger the risk of flood damage. It means then the higher the design water level, the larger the risk of disaster ironically.
  • Keeping this fact in mind, we should make adaptive solutions against wide range of possibilities, including catastrophic event, environmental impact, social/cultural acceptability, and changing climate under various conditions from small to large scales of incidents that can possibly happen. The first protection area, the limited capacity of structure, the emerged area for flooding, and the next event to happen, we need to tackle with those many possibilities. Strategic flood control therefore consists of comprehensive management and is capable only if multiple alternative solutions are prepared in advance. In this sense, the non-structural measures and activities are not separate tasks with structural ones.
  • Finally, as my sincere suggestion, Adaptive, Comprehensive (Multiple) and Sustainable solution should carefully be considered by the river administrators. Measures taken for the time being will further be empowered when other components of the strategic master plan are implemented. And our task is not to prevent flood but change the ‘uncontrollable flooding’ into ‘controlled flooding’.

KAJIAN ALTERNATIF PENANGGULANGAN BANJIR
(STUDI KASUS SUNGAI LADAPA DI KABUPATEN GORONTALO)

Anik Sarminingsih

Sumber:  http://docs.google.com/eprints.undip.ac.id/

 

 

 

 

 

 

 

 

 

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